1. Field of the Invention
The present invention relates to a particle detector and, more particularly, to a particle detector employing an electrical detection zone method for measuring the number and size of particles such as of fine ceramic powder, a pigment or cosmetic powder, wherein the number and size of particles contained in a particle containing liquid are measured on the basis of a change in electric impedance detected when the particle containing liquid is caused to pass through a through-hole.
2. Description of the Related Art
A particle detector known in relation to the present invention comprises: a detection block having a through-hole for particle detection; a first cell which supplies a particle containing liquid enclosed in a sheath liquid into the through-hole; a second cell which receives and discharges the particle containing liquid and the sheath liquid having passed through the through-hole; electrodes respectively provided in the first and second cells; and a slidable member slidably supporting one of the first and second cells so that a distance between the first and second cells can be changed; wherein the detection block is removably held between the first and second cells to liquid-tightly connect the first and second cells to each other (see, for example, Japanese Unexamined Patent Publication No. 2001-33378).
Further, there is known a noise-shielding rack for electronic devices, which has a double structure including an inner body, an outer body and an insulative member interposed therebetween, wherein the inner body directly houses an electronic device and the outer body is grounded (see, for example, Japanese Unexamined Patent Publication No. HEI11(1999)-87981).
Conventionally, an electrical detection zone method is employed for measuring the number and size of blood cells in blood, or particles such as cement powder, latex or toner for industrial use. In the electrical detection zone method, a partition having a single through-hole is provided in an electrolytic solution, and two electrodes are disposed on opposite sides of the through-hole. Particles to be subjected to the measurement are dispersed in the electrolytic solution, and the resulting particle containing liquid is caused to flow through the through-hole.
When the particles pass through the through-hole, an electrical resistance is instantaneously changed, and voltage pulses are generated between the electrodes. The height ΔV of each of the pulses reflects on the volume Vp of a particle as expressed by the following expression:ΔV=I·ρ·Vp/S2  (1) (wherein I is a constant electric current flowing between the electrodes, S is a sectional area of the through-hole, and ρ is the electrical resistance of the electrolytic solution). Therefore, the sphere equivalent diameter of the particle can be determined irrespective of the shape of the particle. As a result, the volume-based size of the particle can be determined. Further, the number of the particles can be determined on the basis of the number of the pulses.
However, the conventional particle detector has the following drawbacks.    (1) The electrical detection zone method suffers from problems such that the intensity of a detection signal varies depending on the position of the particle within the through-hole through which the particle passes, that plural particles passing close to each other are counted as one, and that particles having passed through the through-hole stagnate around the through-hole to cause noises. A conventional approach to these problems is to employ a sheath flow method in combination with the electrical detection zone method. In the combination sheath flow method, a stream of a particle containing liquid is enclosed in another liquid (sheath liquid) in a flow cell so as to be narrowed, whereby the particles can be introduced in line into the through-hole along the center axis of the through-hole. Thus, the particle size can be determined with minimum errors. However, it is essential to accurately align the axis of the narrowed stream of the particle containing liquid with the center axis of the through-hole for highly accurate detection in a particle detector based on such a principle. Therefore, how to simplify the construction of the particle detector for the accurate alignment is a problem.    (2) In the particle detector employing the electrical detection zone method, i.e., in the particle detector of electrical resistance type, air bubbles are liable to occur in and around the through-hole when the particle containing liquid flows through the through-hole. If the amount of the air bubbles increases, detection pulses between the electrodes are disturbed, so that information on the particles is erroneously detected.    (3) During repeated particle detection, minute substances such as particle pieces are deposited in the through-hole, so that a sectional area S of the through-hole is varied. With the variation in the sectional area S, pulse heights ΔV detected for particles having the same volume Vp differ as can be understood from the expression (1). Therefore, it is difficult to continuously provide detection results with acceptable reproducibility.    (4) In the particle detector of electrical resistance type, noises in the voltage pulses obtained when the particles pass through the through-hole are liable to be enhanced under an influence of external electromagnetic noises. This may reduce the detection accuracy. In recent years, therefore, the European Union (EU) has required medical electronic measuring apparatuses to conform with standards specified by the EMC regulations, i.e., to ensure that measurements are not influenced by radio waves having a specified field intensity.